In recent years, the popularity of systems using wireless radio communication has increased substantially. For example, cellular communication systems and wireless networks have now become commonplace. The increased requirement for frequency spectrum resource has led to an increased desire for efficient communication and especially at higher frequencies and for higher data rates.
For example, Wireless Local Area Networks WLANs are becoming common not only in business environments but also in domestic environments. In order to increase the capacity of such WLANs, it is desirable to increase the data rate of the wireless communication. As a specific example, the Institute of Electrical and Electronic Engineers (IEEE) have formed a committee for standardising a high-speed WLAN standard known as IEEE 802.11n(TradeMark). The 802.11n™ standard comprises Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications aimed at enhancing WLANs to provide higher effective data throughputs. It is intended that the 802.11n™ standard will help WLANs meet the expanding bandwidth needs of enterprise and home networks, as well as those of WLAN hot spots.
In order to achieve high data rates over the air interface, a number of advanced radio techniques are employed. It has been found that in systems using open-loop approaches (i.e. without the transmitter using knowledge of the transmit channel or the signal received at the receiver) significant improvement can be achieved by using multiple antennas at the transmitter and the receiver. In particular, many radio communication systems such as WLANs provide for a plurality of transmit and receive antennas to be used. Specifically, some transmission techniques involve transmitting a data stream by simultaneously transmitting different signals derived from the data stream from different antennas over the same communication channel. The receiver(s) of these techniques typically also comprise a plurality of antennas each of which receive a combined signal corresponding to the transmitted signals modified by the individual propagation characteristics of the radio link between the individual antennas. The receiver may then retrieve the transmitted data stream by evaluating the received combined signal.
Such techniques are known as Multiple Transmit Multiple Receive (MTMR) schemes and can be designed to derive benefit from spatial diversity between the antennas in order to improve the detection. Indeed, the equivalent Signal to Noise Ratio (SNR) of the combined signal is typically increased compared to the single antenna case thereby allowing higher channel symbol rates or higher order constellations. This may increase the data rate for the communication link and thus the capacity of the communication system.
For the situation where two transmit antennas are used, an efficient transmit case has been proposed in “A simple transmit diversity technique for wireless communications,” by S. M. Alamouti IEEE Journal on Selected Areas in Comm., pp. 1451-1458, October 1998. The proposed technique is known as Alamouti coding and comprises transmitting two symbols from two antennas during two symbol times such that the original symbols may easily be derived by a receiver. The proposed technique is a specific case of Space-Time Block Coding (STBC) and results in high performance for two transmit antennas by providing for orthogonal transmission of the two symbols while fully exploiting the spatial diversity of the two antennas.
However, it is not possible to directly extend the Alamouti coding technique to more than two transmit antennas while providing full orthogonality and fully exploiting the antenna diversity. Accordingly, the performance improvement achieved from adding additional transmit antennas reduce for increasing antenna numbers for STBC techniques. Furthermore, it has been demonstrated that such approaches are sub-optimal from a capacity point of view when the receiver has two or more antennas. Accordingly, STBC techniques are very suitable for systems wherein relatively small numbers of transmit and receive antennas are used but are less suitable for systems using a larger number of antennas.
Other transmit schemes used for multiple transmit antennas consist in simultaneously transmitting different data streams from different antennas to increase the spectral efficiency of the system. For example, a single data stream may be split into two sub-data streams that are then individually and simultaneously transmitted over the same communication channel from different antennas. In these spatial division multiplexing techniques, the receiver evaluates the combined signals received from the transmit antennas and jointly determines the transmitted symbols.
Theoretically, a maximum likelihood approach can be used wherein the data symbols of the different data streams are selected as the data symbols which for the estimated sub-communication channel characteristics most probably would result in the received combined signal. However, such a technique is computationally demanding and is impractical for most systems. Accordingly, other receive techniques including Zero-Forcing and Minimum Mean Square Error (MMSE) techniques are typically used, potentially combined with iterative detection techniques. However, these techniques rely on there being a large number of decorrelated received signal samples available thereby requiring a large number of receive antennas. Thus, such techniques suffer from relatively poor performance in view of the high decoding complexity, and in particular in cases where identical number of transmit and receive antennas are used.
An example of a transmit technique for multiple transmit antennas have been proposed in “Double-STTD scheme for HSDPA systems with four transmit antennas: Link Level Simulation Results” by Texas Instruments presented to the 3rd Generation Partnership Project (3GPP) in TSG-R WG1 document, TSGR1#20(01)0458, 21-24 May, 2001, Busan, Korea.
In this technique, data is transmitted over four transmit antennas by individually Alamouti coding two data streams and simultaneously transmitting each of the resulting data streams over two of the antennas. Hence, two data streams are transmitted simultaneously from 4 antennas, each stream being encoded using the Alamouti code. However, although improved performance may be achieved over a classical four antenna space division multiplexing scheme, the resulting data rate is significantly reduced due to the symbol doubling associated with the Alamouti code.
Hence, an improved wireless communication technique using multiple transmit antennas and preferably multiple receive antennas would be advantageous and in particular a technique allowing for improved performance and/or reduced complexity would be advantageous.